Hitherto, there have been two types of golf balls. One is a solid golf ball, such as a two piece solid golf ball which is composed of a core formed by rubber material and a cover formed from thermoplastic resin (e.g. ionomer resin), surrounding the core. The other is a thread wound golf ball which is composed of a solid or liquid center, a thread layer formed by winding thread rubber on the center and a cover with 1 to 2 mm thick formed from ionomer resin or balata. The two piece solid golf ball, when compared with the thread wound golf ball, has superior flight performance, because it shows higher initial velocity at the time of hitting and therefore obtains longer flight distance. The two piece solid golf ball also shows superior durability to the thread wound golf ball. Accordingly, the two piece solid golf ball has been exclusively used by many golfers, especially amateur golfers. However, the two piece solid golf ball has a defect in that shot feel being hard when hitting.
In order to improve the defect of the solid golf balls, the present inventors have proposed that the core of the solid golf balls is made hollow so that the inertia moment of the golf ball increases to improve shot feel when hitting without injuring the excellent flight performance of the solid golf ball.
The reason why a hollow golf ball attains the above improvement is as follow. The continuance of spin of the two piece solid golf ball is governed not only by the configuration of dimples but also by the moment of inertia of the golf ball. It is believed that the larger the moment of inertia, the more difficult the spin is applied on the golf ball but the more lasting the spin once spin is applied on the golf ball. This means that, if a golf ball has larger moment of inertia, the golf ball, when hitting with a driver, does not receive spin so much and therefore prolongs its flight distance without blowing up by the air. The golf ball generally receives lifting power during flight, but the lifting power is controlled by the amount of spin. The more the spin the stronger the lifting power. Between the hitting point by a driver and the highest point of the golf ball, less spin amount is better to prolong flight distance, because the lifting power would be low if the amount of spin is low and therefore the power of the component returning the ball in the lifting power would be low. After the ball reaches the highest point, a larger spin amount or larger continuance of spin is better to prolong flight distance, because the lifting power would be highly maintained if the amount of spin is highly maintained and therefore the power of the component carrying the ball forward in the lifting power would be high. Accordingly, the larger the moment of inertia of the golf ball, the more the flight distance is prolonged. In addition, when it is used at approach shot, the larger the moment of inertia, the easier the golf ball is controlled, because the back spin once applied on the golf ball would be continuously maintained and the ball would stop on the green to back spin. In view of shot feel, it is believed that the core is made soft and the shot feel is softened, but if the core is made hollow, this leads to the ball being soft.
However, making solid cores hollow leads to other problems in producing the golf balls. For example, the rubber composition for the core is prepared and subjected to vulcanization either by press molding using a mold shown in FIG. 2 in which a core mold 11 containing a core 10 is used, or injection molding, to form a semi-spherical shell 12. Two of the semi-spherical shells are adhered with each other by rubber cement to form a core. The golf ball using the thus obtained core has poor durability, because the core is broken at the surface of the adhering area as the impact force is very high when the ball is hit by a golf club. As another example, a rubber composition for core is prepared and subjected to semi-vulcanization either by press molding using a mold shown in FIG. 2 in which a core mold 11 containing a core 10 is used, or injection molding, to form a semi-vulcanized semi-spherical shell 12. Two of the semi-vulcanized semi-spherical shells are contacted with each other and then vulcanized using a mold shown in FIG. 3 to form a core. In the step of the second vulcanization, the core is deformed by expansion of rubber and the hollow portion is not always complete sphere, thus forming distribution in core thickness. This creates the difference in shot feel and flight performance depending on the portion to be hit by a golf club.